1. Technical Field
The present invention relates to an abnormality detection device and an abnormality detection method for detecting abnormality of a reducing agent injection valve that injects a reducing agent for purifying nitrogen oxides in exhaust gas into the inside of an exhaust pipe and to an internal combustion engine exhaust gas purification system.
The present invention particularly relates to a reducing agent injection valve abnormality detection device and abnormality detection method for precisely detecting clogging occurring in a reducing agent injection valve and to an internal combustion engine exhaust gas purification system.
2. Related Art
Conventionally, nitrogen oxides (NOx) are included in exhaust gas exhausted from internal combustion engines of automobiles and the like. As one exhaust gas purification system that reduces and purifies the NOx, there is an exhaust gas purification system configured such that a NOx purification catalyst is disposed in an exhaust pipe of an internal combustion engine and a reducing agent such as an aqueous solution of urea or unburned fuel is injected on the upstream side of the NOx purification catalyst. This exhaust gas purification system is configured to promote, in the NOx purification catalyst, a reduction reaction between the reducing agent and the NOx in the exhaust gas to thereby break down the NOx into nitrogen, water, and carbon dioxide and release the nitrogen, water, and carbon dioxide into the atmosphere.
As one aspect of a reducing agent injection device with which this exhaust gas purification system is equipped, there is an injection-type reducing agent injection device that supplies the reducing agent directly to the inside of the exhaust pipe via a reducing agent injection valve.
Here, when an aqueous solution of urea is used as the reducing agent, the aqueous solution of urea has the property that it freezes and crystallizes in a predetermined temperature range, and it is easy for the aqueous solution of urea to cause clogging of the reducing agent injection valve. Further, sometimes, regardless of the type of reducing agent, clogging occurs in the tip portion or the inside of the reducing agent injection valve as a result of particulate matter of soot or the like or unburned fuel included in the exhaust gas entering the inside of the reducing agent injection valve and causing seizure. Additionally, when clogging occurs in the reducing agent injection valve, it becomes difficult to inject a target quantity of the reducing agent into the inside of the exhaust pipe, the quantity of the reducing agent that should be supplied to the NOx purification catalyst becomes insufficient, and exhaust gas purification efficiency ends up deteriorating.
Thus, as a method of detecting clogging of the reducing agent injection valve, there has been disclosed a clogging determination method that uses a pressure drop quantity inside a supply path interconnecting the reducing agent injection valve and a pump to determine whether or not there is clogging (see JP-A-2008-180193).
Specifically, this clogging determination method determines, in an injection-type injection device, clogging of the reducing agent injection valve by comparing a pressure drop quantity at a time when the valve is open, which represents a pressure drop quantity inside the supply path whose pressure has dropped in a predetermined amount of time when pressure-feeding by the pump has been stopped in a state where the reducing agent injection valve is in an injection mode, and a pressure drop quantity at a time when the valve is closed, which represents a pressure drop quantity inside the supply path whose pressure has dropped in a predetermined amount of time when pressure-feeding by the pump has been stopped in a state where the reducing agent injection valve is in a completely closed mode.
That is, this clogging determination method determines that clogging has occurred in the reducing agent injection valve when the difference between the pressure drop quantity at the time when the valve is open and the pressure drop quantity at the time when the valve is closed has become equal to or less than a predetermined value.
Further, as a method that uses a pressure drop quantity inside the supply path to determine whether or not there is clogging, there has also been disclosed the method described below (see JP-A-2008-202469).
Specifically, this reducing agent path clogging determination method includes the steps of: controlling the drive duty of the pump such that a value detected by a pressure sensor disposed in a first reducing agent path (a path disposed between the pump and the reducing agent injection valve) is maintained at a predetermined value to thereby discriminate whether or not the drive duty of the pump is less than a predetermined threshold value; stopping the pump and completely opening the reducing agent injection valve when it has been discriminated that the drive duty of the pump is less than the predetermined threshold value to thereby calculate a pressure drop quantity where the pressure value inside the first reducing agent path detected by the pressure sensor drops in a predetermined amount of time; and discriminating whether or not clogging is occurring in the first reducing agent path or a second reducing agent path (a path that is connected to the reducing agent injection valve and is for allowing the reducing agent to circulate) on the basis of the pressure drop quantity that has been calculated.
However, in the clogging determination methods described in JP-A-2008-180193 and JP-A-2008-202469, there is the potential for the pressure drop quantity inside the supply path to become smaller than the pressure drop quantity assumed from the injection quantity of the reducing agent, such that an erroneous determination that clogging is occurring ends up being made.
That is, the clogging determination methods described in JP-A-2008-180193 and JP-A-2008-202469 are based on the pressure drop quantity inside the supply path becoming smaller because, when clogging of the injection hole is occurring, the injection quantity when the reducing agent injection valve has been opened for a predetermined amount of time at a predetermined pressure decreases by an amount corresponding to clogging.
Consequently, determination is performed in a state where the reducing agent is pressurized and pressure-fed to the inside of the supply path and where a liquid component occupies the inside of the supply path, but sometimes air mixes in and remains in the supply path of the reducing agent or air is dissolved in the reducing agent under a pressurized state.
Additionally, usually a gas has a much larger compressibility as compared to a liquid, so a fluctuation in the volume of a gas inside the supply path when a fluctuation in the pressure inside the supply path has occurred in accompaniment with the opening of the reducing agent injection valve becomes extremely large as compared to a fluctuation in the volume of a liquid.
Thus, even if the quantity of a gas existing inside the supply path were a trace quantity, it would become difficult for the pressure to drop to an extent matching the injection quantity of the reducing agent because of an increase in the volume of the gas accompanying a drop in the pressure inside the supply path.
That is, as described above, in the clogging determination methods described in JP-A-2008-180193 and JP-A-2008-202469, there is the potential for an erroneous determination that clogging is occurring to end up being made even though clogging is not actually occurring.
Thus, the inventors of the present invention made every effort to discover that this problem can be solved by using, when performing determination of clogging of the reducing agent injection valve using the pressure value inside the supply path of the reducing agent, the pressure value detected by the pressure sensor in a state where a gas (air, exhaust gas, etc.) occupies the inside of the supply path and performing determination of clogging of the reducing agent injection valve, and thus the inventors completed the present invention.
That is, it is an object of the present invention to provide a reducing agent injection valve abnormality detection device and abnormality detection method that can precisely detect the occurrence of clogging of a reducing agent injection valve and to provide an internal combustion engine exhaust gas purification system.